1. Technical Field
The present patent disclosure relates to a composite dielectric sheet, a method for producing the composite dielectric sheet, and a multilayer electronic component including the composite dielectric sheet. The disclosure particularly relates to the improvement of dielectric properties of the composite dielectric sheet.
2. Background Art
Dielectric sheets are used to manufacture multilayer electronic components such as monolithic capacitors, rolled capacitors (film capacitors), and multilayer wiring boards. The dielectric sheets need to meet various requirements, including those described below.
For the size reduction of the multilayer electronic components, the dielectric sheets need to have large dielectric constants. Furthermore, the dielectric sheets need to have high dielectric strength if the dielectric sheets are used for electronic components, such as capacitors, for use in power supply circuits for power electronics such as fuel cells, wind power generators, and hybrid cars. The power electronics market is expanding because environmental issues are attracting much attention.
On the other hand, the dielectric sheets need to be flexible if the dielectric sheets are used for rolled capacitors or flexible multilayer wiring boards that can follow the shape of surrounding members. Therefore, flexible films made of resin have been used for the above applications instead of the dielectric sheets.
The resin usually has a dielectric constant of about two to five; hence, it is difficult to prepare flexible films having large dielectric constants using the resin.
The following sheets have also been proposed (for example, in Patent Documents 1 to 3, below): flexible composite dielectric sheets which have a large dielectric constant which contain resins and high-dielectric constant fillers, dispersed in the resins, containing high-dielectric constant components such as dielectric ceramic components. Since the composite dielectric sheets are flexible and have large dielectric constants, the composite dielectric sheets can meet requirements for size reduction.
In the composite dielectric sheets, the interfaces between the resins and the fillers are discontinuous and therefore may cause dielectric breakdown. Hence, there is a problem in that the composite dielectric sheets have low dielectric strength. If the fillers are not uniformly dispersed in the resins, the composite dielectric sheets have pores. This causes a problem that the composite dielectric sheets have low dielectric strength.
Patent Document 1 discloses a method for producing a composite dielectric sheet. In the method, a mixture of a molten resin and a filler is formed into a sheet-shaped piece by a dry process. Patent Documents 2 and 3 each disclose another method for producing a composite dielectric sheet by a wet process in which a slurry is prepared by dispersing a filler in an organic solvent solution of a resin and then formed into a sheet-shaped piece, which is dried.
In order to solve the problems regarding low dielectric strength, the sheet formed by the dry process is hot-pressed such that the pores in the sheet are reduced as disclosed in Patent Document 1. It is difficult to uniformly disperse the filler, of which the volume content is 20% or more, in the resin as long as the dry process is used; hence, it is substantially impossible for the composite dielectric sheet to have a dielectric constant of six or more.
The composite dielectric sheet disclosed in Patent Document 2 has a large dielectric constant. Although Patent Document 2 does not describe the dielectric strength of the composite dielectric sheet, the dielectric strength thereof is believed to be low because no measure is taken to eliminate the discontinuity of the resin-filler interfaces.
The composite dielectric sheet disclosed in Patent Document 3 has a large dielectric constant and is prepared in such a manner that the filler is surface-treated with a silane-coupling agent and the slurry is prepared by dispersing the resulting filler in a solution prepared by mixing polyphenylene oxide (PPO), a crosslinking resin, and a reaction initiator and is then formed into such a sheet-shaped piece. PPO has no reactive functional group and therefore has low adhesion with the crosslinking resin. Furthermore, the reaction initiator serves as an impurity. Hence, the dielectric strength of this composite dielectric sheet is believed to be low.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2000-294447    Patent Document 2: Japanese Unexamined Patent Application Publication No. 3-208324    Patent Document 3: Japanese Unexamined Patent Application Publication No. 6-52716